An instrument landing system (ILS) is a ground-based system that provides precision guidance to an aircraft approaching and landing on a runway. The ILS uses Very High Frequency (VHF) and Ultra High Frequency (UHF) guidance signals to enable a safe landing during conditions such as low ceilings or reduced visibility due to fog, rain, or blowing snow. The ILS includes ground-based transmitters that transmit the guidance signals to a receiver on-board the aircraft. The guidance signals produce ILS navigation outputs that are used by the flight control system to bring the aircraft to a safe landing on the runway.
Undetected malfunction of ILS receiver signal processing may result in mission failure that could have catastrophic consequences. Thus, conventional systems have built-in fault detection capabilities. Some on-board ILS receivers detect possible malfunctions using two or more identical receiver paths, coupled to a shared antenna, and tuned to the same radio channel containing the ILS guidance signal broadcasted by the ILS ground station. Each ILS receiver path performs identical, or near-identical, computations on the same input signal to produce the ILS radio guidance data. A malfunction is detected when ILS receiver paths produce different guidance data. The drawback of this approach is the cost associated with employing additional full receiver paths and the reduction in receiver sensitivity associated with splitting the antenna output into multiple inputs for each of the receivers which cuts down on the maximum range at which the guidance signals can be detected.
Another approach favored by the large airplane manufacturers is to feed a single on-board receiver from an antenna to demodulate the desired guidance signals and then digitize the demodulated signal in two parallel digital processing chains to produce the radio guidance data. The processors compare their guidance data to detect any malfunction in the digital processing chain in real-time. To detect malfunctions in the RF down conversion/demodulation, signals whose malfunction may cause erroneous data are also monitored in real-time. In addition a test signal is injected into the RF receiver input during all phases of flight during which the landing guidance signal is not to be received, and signal processors monitor the test signal to detect any malfunction in the RF chain up until the landing final phase of flight is started. The drawback of this approach is that any malfunctions in RF components that cannot be monitored during final approach and landing can result in a loss of guidance.
A method which provides the capability to detect any receiver malfunction in real-time without added parts count and without degrading the receiver sensitivity/maximum detection range would be highly desirable.